The application of moisture sensing elements in modern living are getting more important. The increasing demand, especially in the areas of air conditioning, environmental control and agricultural equipment has shown distinctive growth.
At present, the materials used for humidity sensing are in three categories: (1) electrolyts, (2) fine ceramics and (3) polymers.
The advantages of fine ceramics are high stability and good thermal resistance, but they tend to have narrow measuring temperature range. Furthermcre, the moisture retention due to capillary phenomenon exhibited in ceramics delineates a rather apparent hysteresis.
Polymers show less hystersis, but exhibit inferior thermal resistance. The range of operating temperatures also is quite low, with poor stability.
The principle of humidity sensing in the polymers can be divided into two types; the resistance and capacitance. The capacitance change derived from the change of dielectric constant due to moisture content variation follows the equation shown below:
C: Capacitance PA1 .epsilon.: Dielectric constant PA1 A: Electrodes surface area PA1 D: Distance between two electrodes
Polyimide exhibits excellent heat resisting property and is a hygroscopic insulating material. Its capacitance varies with the dielectric constant due to moisture content variation. Thus, the atmospheric moisture content can be measured.
Because the dielectric constant of polyimide is linearly proportional to its moisture content, consequently, the humidity sensing also exhibits linearity. Furthermore, the excellent thermal resistance of polyimide renders its usefulness recently in the capacitive humidity sensing element which can be found in U.S. Pat. Nos. 4,305,112 and 4,345,301.
For example, U.S. Pat. No. 4,305,112 relates to a capacitive humidity sensing element with a multi-layer structure. It comprises at least a humidity sensing and a moisture permeable, electricity conducting metal layers and these two layers and the substrate are bonded with adhesives. The humidity sensing layer is a soluble polyimide which is derived from imidization. Therefore, the bonding between the layers of polyimide and metal substrate can only be readily achieved by using adhesives. In turn, the promoter used not only interferes with the polyimide hygroscopic property and sensing humidity accuracy, but also increases the complexity of actual manufacturing process as well.
On the other hand, U.S. Pat. No. 4,345,301 relates to a capacitive humidity transducer which comprises two electricity conducting coatings on both sides of polyimide layer as parallel capacitance plates. These two plates are copper coated and both need electrical connectors. Copper electrodes are readily corroded in the presence of moisture. Thus it renders less desirable for long term applications. In addiiton, its construction is more complex than the present invention.